1. Field of the Invention
The present invention relates to a high temperature combustion catalyst and a method for producing the high temperature combustion catalyst and, more particularly, to a high temperature combustion catalyst for use in a catalytic combustion type gas turbine combustor or the like, which possesses a large activity at a high temperature, a superior low temperature ignitionability, a superior thermal durability at a high temperature and a long life, and a method for producing the high temperature combustion catalyst.
2. Description of the Prior Art
Recently, as petroleum resources and the like have been reduced, for example, fuel such as a mixture gas of natural gas or the like has been desired to be burned at a temperature of as high as possible in a gas turbine combustor.
However, conventionally, a mixture of fuel and air is ignited by using a spark plug to burn the mixture, and a temperature within a combustor is partially raised beyond 2000.degree. C. It is well-known that a large amount of nitrogen oxides (NOx) are produced in such a high temperature portion of the combustor, thereby bringing about a problem such as an environmental pollution.
In order to solve the problem of this kind, a catalytic combustion method in which the mixture of the fuel and the air is burned in the presence of a catalyst, has been proposed. In this case, the mixture gas is burned by virtue of a catalytic action of a platinum group noble metal element such as palladium and platinum. As is well-known, a catalytic reaction utilizing the catalytic action of the noble metal element can be effectively proceeded at a low temperature such as about 300.degree. C. when a sufficient active surface area is ensured. Accordingly, in this catalytic reaction method, the combustion can be initiated at a relatively low temperature and the combustion temperature is gradually raised with no maximal value. In this case, the maximum temperature becomes relatively low such as approximately 1500.degree. C. Hence, when a gas including an oxidizing gas such as the air including nitrogen gas is used for the combustion, the production of the nitrogen oxides can be extremely reduced, and, in the same time, a deterioration of the combustor itself by heat can be prevented.
In general, the noble metal element carried on a carrier is used as a catalytic component. For instance, on a surface of a cordierite carrier substrate having a honey comb shape, an alumina carrier layer for increasing an effective surface area is formed to obtain a carrier body, and then the minute catalyst particles of the noble metal element are carried on the alumina carrier layer of the carrier body to obtain a catalyst. In this case, as the particle size of the noble metal element to be carried on the carrier body is smaller, the low temperature ignitionability of the catalyst is improved much better. However, the heat-resistant temperature of the catalyst is rather low such as below 500.degree. C., for instance, in the temperature range of more than 600.degree. C., the noble metal particles carried on the alumina carrier layer start to aggregate, and the active surface area of the catalyst is reduced, with the result of the lowering of the activity of the catalyst. Further, this phenomenon more remarkably appears as the particle size of the noble metal element becomes smaller.
Further, conventionally, for example, .gamma.-alumina is applied as an active carrier layer onto a carrier substrate having a certain mechanical strength, and then the noble metal element is carried on the carrier body by an immersion method to obtain a combustion catalyst. However, in this noble metal combustion catalyst, it is said that usually the heat-resistant temperature of the catalyst is approximately 600.degree. C. Therefore, the catalytic activity of this catalyst is rapidly fallen down at more than 600.degree. C. and thus the catalyst cannot be used any more. For the reasons, it is considered that, first, the noble metal particles migrate and aggregate at the high temperature and hence the surface area of the catalyst is reduced, thereby lowering the combustion efficiency of the catalyst, that, second, a phase transition of .gamma.-alumina to .gamma.-alumina is taken place at around 1000.degree. C. or more and thus cracks are produced in the alumina carrier layer or between the alumina carrier layer and the carrier substrate, resulting in peeling off the alumina carrier layer together with the catalytic noble metals, and that, third, at around 1000.degree. C., the alumina itself is sintered and pores serving as passages for the gas in the alumina carrier layer are collapsed, thereby burying the catalyst particles in the carrier layer and preventing the catalyst particles from contacting wih the gas.
Then, in order to improve the thermal durability of the noble metal combustion catalyst, the .gamma.-alumina layer is so improved that platinum particles on the .gamma.-alumina layer are strongly adsorbed onto the .gamma.-alumina layer for preventing the migration and the aggregation of the platinum particles at a high temperature and that the phase transition of the .gamma.-alumina layer to the .gamma.-alumina is prevented for preventing the above-described cracks. For instance, there has been a conventional catalyst further including a rare earth element carried on an alumina carrier layer in order to prevent a decrease of its surface are due to the combustion heat. In this case, the thermal durability at a high temperature is superior as compared with the aforementioned conventional catalysts, but the thermal durability is incompatible with the low temperature ignitionability at the high temperature in a gas turbine combustor or the like, which is impractical.
Then, in order to solve this problem, other catalysts for a gas turbine combustor further including nickel carried on the alumina carrier layer has been proposed, as disclosed in the Japanese Patent Laying-Open Specifications Nos. 60-19651 and 61-28455. However, in these cases, it is still difficult to simultaneously satisfy the low temperature ignitionability and the thermal durability at the high temperature, and thus they should be further improved.